Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m.

Yes, it looks like 80m per leg is reasonable - 4 wavelengths at 20m
I did as you suggested in terms of modelling the sag and it didn't seem
to upset the pattern greatly.

I can put the antenna over the house but I'm not sure if the extra
feeder loss would outweigh the advantages. Might not be too bad with 600
ohm or greater feeder.
Alan